Self-healing materials that can repair cracks and other damage automatically have the subject of research for decades. Now a team of scientists at Rice University have come up with a new twist. It's a Self-Adaptive Composite (SAC) that is not only self healing, but also has reversible self-stiffening properties that allow it to spring back into shape like a sponge.

Until now, self-healing materials have tended to be fairly rigid substances laced with microchannels or capsules filled with resins that squirt out and harden around damaged areas after they've been cracked or punctured. The problem is that this rigidity limits their applications as well as their ability to self-repair. Seeking something that was more a biocompatible material for tissue engineering or that can act as a lightweight, defect-tolerant structural component, the Rice team hit on the idea of a new material based on the structure of living tissue.

"We wanted a biomimetic material that could change itself, or its inner structure, to adapt to external stimulation and thought introducing more liquid would be a way," says Pei Dong, a postdoctoral researcher. "But we wanted the liquid to be stable instead of flowing everywhere."

Microscopic view of the SAC(Credit: Rice University)

What they came up with was a substance made up of two dry polymers mixed with a solvent. When the solvent is evaporated away under heat, the result is a porous mass of gooey spheres that's sort of like a sponge with liquid held tightly inside of it that can be squeezed and then bounce back into shape.

This ability is made possible by sticky, micron-scale rubber balls that make up the solid matrix and can slide against one another under pressure. The resilient balls are made of polyvinylidene fluoride (PVDF) and coated with viscous polydimethylsiloxane (PDMS) that both bind them together and allow them to slide without breaking contact. The liquid inside makes the SAC viscous and elastic, so if it's stretched or compressed, the matrix returns its original state.

According to the Rice team, the SAC acts like a gel, but it's hard, flexible, and dry despite its high liquid content. By forming the two polymers that make up the substance into a reconfigurable matrix, the material's self-stiffening behavior increases by up to 683 percent.

So far, the team says that only small amounts of the new material have been created, but they see no reason why it cannot be scaled up during future development.